The present invention relates to a magneto-rheological (MR) fluid damper, and more particularly, to a linearly-acting MR fluid damper suitable for vibration damping in a vehicle suspension system.
MR fluids are materials that respond to an applied magnetic field with a change in rheological behavior (i.e., change in formation and material flow characteristics). The flow characteristics of these non-Newtonian MR fluids change several orders of magnitude within milliseconds when subjected to a suitable magnetic field. In particular, magnetic particles noncolloidally suspended in fluid align in chain-like structures parallel to the applied magnetic field, changing the shear stress exerted on adjacent shear surfaces.
Devices such as controllable dampers benefit from the controllable shear stress of MR fluid. For example, linearly-acting MR fluid dampers are used in vehicle suspension systems as vibration dampers. At low levels of vehicle vibration, the MR fluid damper lightly damps the vibration, providing a more comfortable ride, by applying a low magnetic field or no magnetic field at all to the MR fluid. At high levels of vehicle vibration, the amount of damping can be selectively increased by increasing the applied magnetic field. The controllable damper lends itself to integration in vehicle suspension systems that respond to vehicle load, road surface condition, and driver preference for a stiffer suspension performance.
In some applications, linearly-acting MR fluid dampers use a piston assembly that moves within a cylinder providing a reservoir of MR fluid. A piston assembly disposed within the cylinder separates the reservoir into a compression chamber and an extension chamber. The piston assembly has a piston core positioned within a flux ring to form an annular flow gap therebetween. Relative motion between the damper body tube and the piston assembly is dampened by a flow of the MR fluid through the flow gap from one chamber to another caused by the relative motion.
Alignment of the flux ring is critical for optimum performance. Ideally, the piston assembly should move freely within the reservoir in the damper body tube without friction or binding. In addition, the radial width and concentricity of the annular flow passage must be precisely set and maintained along the axial length of the passage throughout the operation to ensure optimum, predictable control of the damping. Consequently, the flux ring must be correctly aligned with the piston core.
Attachment elements have been suggested to provide flux ring alignment with nonmagnetic bridge elements. In particular, perforated end plates are aligned above and below the flux ring and piston core. These attachment elements have several potential problems. First, the attachment elements increase the length of the piston assembly. Consequently, less travel distance is available for the piston to move within the cylindrical reservoir of the damper body tube. Second, the attachment elements require tight manufacturing tolerances in order to correctly align the flux ring to the piston core. Third, such attachment elements often include tabs or other projections that increase the drag as the piston moves, which may undesirable. Fourth, the attachments elements have numerous components and require manufacturing operations such as spot welding. Therefore, such attachment elements are costly to manufacture and time consuming to assemble.
Consequently, there is a need for an improved piston assembly suitable for use in a magneto-rheological (MR) fluid damper.
The present invention provides an improved piston assembly for use in an MR fluid damper that provides increased performance. Further, the piston assembly of the present invention provides greater damping capability for a given length of piston. In addition, with the piston assembly of the present invention, the part count of the piston assembly is reduced; and the piston assembly is easier to assemble in a desired alignment. Thus, the piston assembly of the present invention is of a simpler construction than known damper pistons, can be manufactured for less cost and provides an MR fluid damper having better performance without a substantial increase in cost or weight.
According to the principles of the present invention and in accordance with the described embodiment, the present invention provides an MR fluid damper having an annular flux ring assembly surrounding a piston core piston assembly and disposed for reciprocal movement in a cylinder. The flux ring assembly has first and second ferromagnetic flux rings forming opposite ends of the flux ring assembly. A nonmagnetic annular spacer is interposed between the first and second ferromagnetic flux rings. The annular spacer has a plurality of first projecting members extending between the piston core and the flux ring assembly which align the piston core concentrically with respect to the flux ring assembly, thereby forming an annular, first flow path between the piston core and the annular flux ring.
In another aspect, the present invention provides a plurality of second projecting members extending between the flux ring assembly and the cylinder to align the flux ring assembly concentrically with respect to the cylinder, thereby forming an annular, second flow path between the flux ring and the cylinder.
The second projecting members are also relatively inexpensive and simple structures that not only provide the desired concentric alignment but also function as interrupted bearings between the cylinder and the flux ring assembly. In addition, the second flow path provides additional shear area and hence, damping effect, for a given length of the piston assembly.
These and other objects and advantages of the present invention will become more readily apparent during the following detailed description taken in conjunction with the drawings herein.